1. Field of the Invention
The present invention relates to an electrically connecting structure and a method of electric connection. More specifically, the invention relates to an electrical connection using an electrically conducting adhesive.
2. Description of the Related Art
Referring, for example, to Japanese Unexamined Patent Publication (Kokai) No. 52-70369, a connection terminal portion of a substrate provided on a glass substrate which is a liquid crystal panel and a terminal portion of an external circuit substrate are electrically connected together by disposing an electrically conducting rubber between the connection terminal portion of the substrate and the terminal portion of the substrate for connection to an external circuit, involves problems in regard to the assembling operation efficiency since the electrically conducting rubber must be incorporated at a specified position and lack of stability in the contact between the electrically conducting material in the electrically conducting rubber and the terminal portions.
In order to solve the above-mentioned problems, therefore, there has been proposed technology which uses an anisotropic electrically conducting adhesive containing electrically conducting particles instead of using an electrically conducting rubber.
As for means of using an isotropic electrically conducting adhesive disclosed in the aforementioned Japanese Unexamined Patent Publication (Kokai) No. 52-70369, furthermore, Japanese Unexamined Patent Publication (Kokai) No. 58-115779 discloses, by way of numerical figures, a concrete example of electrically connecting a connection terminal portion of a substrate and a terminal portion of an external circuit substrate by using an anisotropic electrically conducting film.
In order to impart electric conduction to parts having low heat resistance such as liquid crystal panels and the like, an anisotropic electrically conducting adhesive has been much used in recent years as taught by the aforementioned prior art. Referring, for instance, to FIGS. 7 and 8 which are diagrams explaining the prior art, electrodes 33 and electrodes 41 are formed on the facing surfaces of a pair of substrates 30 and 40 that are disposed in facing manner, a connection terminal portion 31 of the electrodes 33 and a connection terminal portion (not shown) of the electrodes 41 are disposed on the outside of a sealing portion 50, and liquid crystals are sealed inside the sealing portion 50 to constitute a liquid crystal display device.
A prior art will now be explained with reference to FIGS. 8 and 9 which are views of a liquid crystal display device along the line A--A of FIG. 7. In FIGS. 8 and 9 which are sectional views along the line A--A of FIG. 7 of prior art, on the upper surface of the substrate 32 constituting the liquid crystal display device is disposed a connection terminal portion 31 that electrically couples the liquid crystal panel and the external circuit together, and the electrode composed of an indium tin oxide (ITO) film is disposed on the connection terminal portion 31 and on the pixel unit of the liquid crystal panel in which the liquid crystals are filled. Furthermore, a film-like electrically conducting sheet (hereinafter referred to as FPC) 10 is arranged facing the connection terminal portion 31 of the substrate 30.
Note that the FPC is usually formed in a film like configuration and thus it is generally used as an adhesive material, by being inserted into a space formed between two opposite electrodes. The FPC 10 is obtained by forming a copper pattern 13 which is an electrically conducting wiring pattern of copper or the like on a base film composed of a polyimide or the like, and applying a film of solder or tin of a thickness of 4 to 6 .mu.m onto the copper pattern 13 by electroplating. A portion constituted by the electrically conducting film 14 and the copper pattern 13 of FPC 10 facing the connection terminal portion 31 of the substrate 32 is referred to as connection terminal portion 11. The connection terminal portion 11 and the connection terminal portion 31 are disposed facing each other with an anisotropic electrically conducting adhesive 20 which is an anisotropic electrically conducting film being disposed therebetween. The anisotropic electrically conducting adhesive 20 is constituted mainly by electrically conducting particles 21 and a resin binder 22. In general, the electrically conducting particles 21 comprise beads made of epoxy resin or polystyrene resin, and plated with nickel and gold, and the resin 22, in general, is an epoxy resin.
When a conventional anisotropic electrically conducting adhesive, e.g., CP7131, produced by Sony Chemical Co. is used, the electrically-conducting particles having an average diameter of about 6 .mu.m, and the thickness of the anisotropic electrically conducting adhesive 20 before being adhered with the application of heat and pressure is about 25 .mu.m. On the other hand, the copper pattern 13 at the terminal portion 11 disposed on the FPC 10 has a thickness of 35 .mu.m, and the electrically conducting film 14 formed on the copper pattern 13 has a thickness of about 6 .mu.m. Moreover, the connection terminal portion 33 disposed on the substrate 30 is composed of ITO.
Here, the anisotropic electrically conducting adhesive 20 that has heretofore been used for accomplishing electric connection comprises, for example, an epoxy resin as a main component, the epoxy resin being mixed with capsules containing a suitable curing agent and being further mixed with hard electrically conducting particles such as the aforementioned resin beads. The anisotropic electrically conducting agent 20 is filled in a gap between the two facing connection portions, for example, between the electrodes. When the anisotropic electrically conducting agent 20 is treated with the application of heat of a temperature of, for example, 170 to 180.degree. C. and a pressure of, for example, 40 kg/cm.sup.2 in a direction in which the length of gap formed between the pair of facing electrodes decreases, then, the capsules are destroyed, the curing agent is cured upon reacting with the epoxy resin that is the chief component of the adhesive agent. As a result, the electrically conducting particles and the two electrodes are connected to each other, establishing the electric connection in only a direction in which the pressure is applied.
When the thermosetting resin such as the epoxy resin or the like is cured, thermal energy or ultra-violet radiation can also be used so as to cure the thermosetting resin, instead of the above-mentioned capsules.
According to the above-mentioned prior art as shown in FIG. 8, adhesion is accomplished with the application of heat and pressure from the direction of arrow Y in order to accomplish the electric conduction between the connection terminal portion 31 of the substrate 30 and the connection terminal portion 11 of the FPC 10 via the electrically conducting particles 21 and to firmly adhere the FPC 10 and the substrate 30 together. Being adhered with the application of heat and pressure, the resin moves toward the direction where the internal pressure is low between the FPC 10 and the substrate 30. Here, the electrically conducting particles 21 have poor fluidity and change their positions little and remain between the electrically conducting film 14 of the connection terminal portion 11 and the ITO 33 disposed on the connection terminal portion 31 thereby to establish electric conduction between the electrically conducting film 14 and the ITO 33. However, the present inventors have found the fact that when the adhesion is accomplished with the application of heat and pressure from the direction of arrow Y in FIG. 8, the electrically conducting particles 21 are buried in the electrically conducting film 14 as shown in FIG. 9 since the electrically conducting film 14 of FPC 10 is composed of a solder or a tin film having a thickness of about 6 .mu.m, the electrically conducting particles 21 have an outer diameter of about 6 .mu.m, and the ITO 33 of the substrate 10 is composed of indite tin oxide. Accordingly, the electrically conducting particles lose their action to a conspicuous decree, whereby the conduction resistance increases and Door conduction occurs causing the conduction to be interrupted.
When a connection terminal portion 20 of a data processing circuit 40, for example a liquid crystal display device, including electrodes 33 and 41 and a suitable connection terminal 10 connected to an external control circuit (not shown) are connected together according to the prior art as shown in FIGS. 7 to 9, a digital control signal is fed from the control circuit to the connection terminal portion 20 of the data processing circuit 40 via the connection terminal 10, and ON or OFF level of the digital signal can be discriminated even when the connection resistance of the connection portion is high to some extent. In the field of liquid crystal technology in recent years, however, a data transfer device has been developed in which a data processing circuit and a drive circuit means for driving the data processing circuit are formed on the same glass substrate. In such a data transfer device, a signal for controlling the drive circuit means is, in many cases, an analog signal that is input from a control circuit outside the glass substrate via wiring.
In the above-mentioned data transfer device, therefore, it becomes difficult to reliably and correctly discriminate a change in the analog signal unless the resistance is decreased in the connection portion between the control circuit and the drive circuit means. It has, therefore, been desired to decrease the connection resistance.
On the other hand, in a liquid crystal display panel called a chip on glass (COG) type panel, in which an integral circuit (IC), for driving the liquid crystal display panel, is mounted on a transparent substrate of the panel, electrical power for driving the IC should be applied to the connecting terminal portions and thus a large amount of current may flow through the connecting terminal portions connected to the power source. Therefore, it is naturally required that the connection resistance should be reduce as much as possible.